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Transcript
Nuclear Chemistry The weird world of the nucleus Isotope Refresher Atoms of the same element all have the same number of protons Atoms of the same element may have different neutrons and therefore mass numbers (p+ + n0) 235 92 U Nuclear Instability Not all combinations of protons and neutrons are created equal Some are more unstable than others. If they are unstable they will do one of the following: Radioactive decay Nuclear fission Nuclear fusion Nuclear “Reactions” Nuclear “reactions” must still be balanced. U He Th 238 92 4 2 234 90 Notice 238 = 4 + 234 “Mass balanced” Notice 92 = 2 + 90 “Charge balanced” Radioactive Decay Radioactive Decay Radioactive decay – the nucleus of an atom undergoes a change so that it is no longer the same element Decay is a totally random event. Nothing has an effect when an atom decays Two Main Types of Radioactive Decay 1. Alpha decay 2. Beta decay Alpha Decay Alpha decay – emission of an alpha particle from the nucleus Alpha (α) particle - a helium-4 nucleus 4 2 He Uranium-235, “enriched uranium,” decays by alpha decay U He Th 235 92 4 2 231 90 Beta Decay Beta decay – emission of a beta particle from the nucleus 0 Beta (β) particle – electron 1 e Thorium-231 decays by beta decay 0 1 Th e Pa 231 90 231 91 Practice Decay Reactions Beta decay by cesium-137 0 1 Cs e Ba 137 55 137 56 Alpha decay by polonium-210 210 84 Po He Pb 4 2 206 82 Decay Series Some atoms require multiple decays to become stable Most decay series of naturally occurring isotopes end in lead. Half Life Half Life Half life – the amount of time it takes for ½ of a radioactive isotope to decay into something else. Notice the atoms don’t disappear, they just change their identity. Half Life Problem A 100.0g radioactive sample decays for 5 hours. Only 12.5g of the original isotope remains after 5 hours. How long is the half life? 3 half lives = 5 hours 5hours/3half lives = 1.67hours/halflife Number of half lives Mass of sample Time 0 100.0g 0 1 50.0g 2 25.0g 3 12.5g 5 hours Half Life Problem Selenium-72 has a half life of 8.40 days. How much of a 450.0g sample of selenium-72 will remain after 42.0 days? Number of half lives Mass of sample Time 0 450.0g 0 1 225.0g 8.40 days 2 112.5g 16.8 days 3 56.25g 33.6 days 25.2 days 4 28.13g 33.6 days 5 14.06g 42.0 days Radiocarbon Dating Technique Uses the known half life of C-14 to estimate death of organic matter Based on the known ratio of C-14 to C-12 Nuclear Fission and Fusion Nuclear Fission Nuclear fission – one atom’s nucleus splits apart. U n Ba Kr 3 n 235 92 1 0 141 56 92 36 1 0 A neutron strikes a nucleus causing it to split into small pieces Releases lots of energy. Extra neutrons are also produced. Nuclear Fission The extra neutrons can cause a chain reaction Uses of Nuclear Fission Nuclear power Energy produced by fission is transferred as heat to a coolant Nuclear Power Uses of Nuclear Fission Mushroom cloud from Nagasaki Uses of Nuclear Fission Nuclear weaponry “Atomic bomb” “A-bomb” Runaway fission reaction resulting in an explosion. Nuclear Fusion Fusion – combining of nuclei 0 1 1 1 H H H e 2 1 H H He 1 1 3 2 2 1 1 1 3 2 He He He H H 3 2 4 2 1 1 1 1 Above reactions describe fusion in the Sun. Releases more energy than fission. Also requires very high temperatures. Uses of Nuclear Fusion Uses of Nuclear Fusion Fusion weapons Hydrogen bomb “H-bomb” Thermonuclear bomb Uses a fission reaction to start the fusion reaction “A-bomb” is the detonator for an “H-bomb” Radiation 3 Primary Types of Radiation Alpha radiation – α – alpha particles 1. Low energy Result of alpha decay Low penetration Stop with a sheet of paper Beta radiation – β –beta particles (electrons) 2. Higher energy Result of beta decay Some penetration ability Stop with several sheets of aluminum foil Gamma radiation – γ –high energy gamma rays 3. Very high energy Can result from any type of decay Very highly penetrating and damaging Stop with several centimeters of lead or very thick concrete 3 Primary Types of Radiation Sources of Human Exposure